Method and apparatus for encapsulating electronic components

ABSTRACT

A centrifuge (18) is used to introduce a fluid, particularly a viscous fluid such as an encapsulating resin, into a cavity in a container (10), particularly a small cavity such as the inside of an electronic component housing. Measured quantities of the fluid are placed in dispensing vessels (28) in the centrifuge opposite the containers to be filled. The dispensing vessels have openings (36) through which the fluid can flow when centrifugal force is applied. On the application of this force, all the fluid flows out of the vessels into the containers.

This invention relates to a method of encapsulating electroniccomponents and also to an apparatus by which such components can beencapsulated. The invention extends to filling, casting, embedding,encapsulation, impregnation and potting applications. In suchapplications, typically a relatively high viscosity liquid resin is tobe introduced into a small cavity, and a particular use for which theinvention is envisaged is the introduction of encapsulating resins intohousings for electronic components.

It is known to make use of centrifugal force to assist the filling ofmould cavities with fluid materials, for example molten metals orplastics. Such processes are known as "spin casting" or "centrifugalcasting".

It is also known from British Patent Specification 2 241 465 to usecentrifugal force to assist the filling of components with encapsulatingresin. However in this specification an overfilling technique isdescribed, and the top face of the components is closed during filling.This has the disadvantage that it is necessary to snap off an injectionnozzle from the filled and cured component.

In prior art processes, a certain quantity of resin or other liquidremains in and hardens in a feed channel and is waste which isdiscarded.

According to the present invention there is provided a method ofencapsulating components in an encapsulating liquid, the methodcomprising the steps of

a locating components in respective chambers, each chamber having wallsdefining a cavity, and an opening,

b providing a separate dispensing vessel for each chamber,

c placing into each dispensing vessel the quantity of liquid required toencapsulate the components in the associated chamber,

d placing the chambers and the dispensing vessels into a centrifuge sothat the opening of each chamber, when the centrifuge is operating, liesradially outside of a dispensing opening of the dispensing vessel, withthe chamber walls surrounding each cavity opening being out of contactwith the dispensing vessel,

e evacuating the centrifuge so that air is evacuated from the cavitybefore the liquid is introduced, and

f operating the centrifuge so that all the liquid flows out of thedispensing opening, into the cavity and around the component throughcentrifugal force.

By charging the dispensing vessel with the exact quantity of liquid tobe introduced into the cavity, the advantage is achieved that no liquidis wasted. This is of particular benefit when the liquid is a resinwhich cures, because once curing has taken place, recycling of the resinis no longer possible. Furthermore by suitably designing the dispensingvessel and its opening, it is possible to ensure that after operationthe dispensing vessel needs no cleaning before being refilled with thenext charge of resin.

In the encapsulation of electronic components by a settable resin, it isimportant to ensure that no air is trapped in the resin and/or in thecavity. Preferably therefore the resin to be introduced into the cavityis degassed before being placed in the dispensing vessel.

According to a second aspect of the invention there is providedapparatus for encapsulating components in an encapsulating liquid, theapparatus comprising a plurality of dispensing vessels each having adispensing opening, the dimensions of the dispensing openings being suchthat encapsulating liquid will not flow therethrough under normalgravitational conditions but will flow therethrough when centrifugalforce is applied, a vacuum centrifuge, chambers defining cavities withinwhich the components will be encapsulated, and means for locating thedispensing vessels and the chambers in the vacuum centrifuge so thatwhen the centrifuge is operating the cavities are evacuated and arelocated radially outward of, and out of contact with, the dispensingopenings of the dispensing vessels such that when the liquid is drawnout of the dispensing openings on application of centrifugal forcethrough centrifuge operation, the liquid will flow into the cavities andaround the components in the cavities to encapsulate the components.

Conveniently the dispensing vessel comprises a cup with an open top anda narrow bore in its base to form the dispensing opening. A measuredquantity of liquid resin can be placed in the cup through the open top,and will be prevented from flowing out through the opening under normalgravitational conditions by surface tension or suitable rheology.

In a preferred embodiment the dispensing vessels and the chamber supportlocations are mounted together in swing-out buckets so that the opentops of the dispensing vessels are uppermost until the centrifuge startsto operate, so that there is no danger of the liquid resin running outof the open top of the dispensing cups before the centrifuge starts tooperate. However when the resin is particularly viscous or thixotropicit may be possible to use dispensing vessels which are radially directedat all times, rather than only during centrifuge operation.

Preferably the step of measuring the quantity of liquid resin to beintroduced into the dispensing vessel takes place under atmosphericconditions and the centrifuging operation takes place under vacuumconditions.

The invention will now be further described, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is a perspective view of an electronic component to beencapsulated;

FIG. 2 is a schematic view of apparatus in accordance with theinvention;

FIG. 3 is a detailed perspective view of part of the apparatus of FIG.2;

FIGS. 4a, 4b and 4c are sections through the apparatus of FIG. 3,showing three different stages in the operation of the apparatus;

FIGS. 5a, 5b, 5c, 5d and 5e show five sequential stages in the operationof an alternative form of apparatus in accordance with the invention;

FIGS. 6a and 6b show one method of measuring the quantity of liquidplaced in the dispensing vessel; and

FIGS. 7a, 7b and 7c show three sequential stages in the operation of athird embodiment of apparatus in accordance with the invention.

FIG. 1 shows an electronic component 10, before encapsulation. Thecomponent comprises an open topped housing 12 in which a toroidal, woundmagnetic core 14 is placed. The windings around the core 14 areconnected to component terminals 16. The component 10 as shown in FIG. 1is operatively complete, but it is normal to fill the space within thecasing 12 with an encapsulating resin to encapsulate the core 14 toprotect the component and to fill all internal interstices withinsulation.

The invention is in no way restricted by the nature of the contents ofthe housing 12.

The principle of encapsulation of such components is well known. Howeverit is often difficult to perform satisfactory encapsulation,particularly with very small components. The resins used forencapsulation (typically filled epoxy resins although the man skilled inthe art will be aware of the wide variety of resins which can be usedfor this purpose) are viscous and do not flow easily into smallcavities. It is also necessary to ensure that the terminals 16 are keptclear of resin.

FIG. 2 shows a centrifuge generally designated 18 with a chamber 20 inwhich a vacuum can be maintained. The centrifuge has a rotor 22 and ateach end of this rotor a frame 24 is mounted. When the rotor 22 rotates,the frames 24 are subjected to centrifugal force so that componentsmounted in the frame can be filled with resin, as will be described inthe following.

FIG. 3 shows one of these cages 24 on a larger scale. The cage has aninner plate 26 in which a plurality of dispensing vessels 28 are formed.The cage also has an outer plate 30 with a mounting track 32 on which aplurality of components 10 are mounted. The components 10 are accuratelypositioned on a radial line of the centrifuge rotor passing through thecentre of each of the dispensing-vessels 28.

In operation, resin 38 mixed with catalyst is charged into all thedispensing vessels 28 (see FIG. 4a). Each dispensing vessel has acup-like chamber 34 and a narrow neck 36 which forms a dispensingopening. The rheology of the resin 38 in the cup 34 is designed to besufficient to ensure that the resin does not drip out of the cup whenthe cup is oriented as shown in FIGS. 4.

When the centrifuge operates and the cage 24 is spun, the resin isforced out through the opening 36 under centrifugal force and enters thecomponent 10 as shown in FIG. 4b. When all the resin has passed out ofthe cup 34 into the component, the component will be completely filledas shown in FIG. 4c and the quantity of resin initially charged into thecup 34 will be the same as that required to fill the component 10 to thedesired level.

An apparatus as described is satisfactory for use where the resin has ahigh viscosity or thixotropy. However if the rheology of the resin issuch that there might be a tendency for the resin to run out of the cupor component when the centrifuge has insufficient rotational speed, thecup and the associated component may be mounted together in a "swing-outbucket" arrangement as shown in FIGS. 5. FIG. 5a shows two cups 34a and34b located vertically above two components 10a and 10b. In thisposition a force equal to gravity acts on the mass of the resin, andthis force is insufficient to cause the resin to flow through thedispensing opening 36.

Alternatively, if the rheology of the resin and the shape of the cup andcomponent are such that the resin will be retained in the component bysurface tension, but might run out of the cup, then the cup can befitted with a cap to retain the resin until centrifugal force isapplied. If the centrifuging step is carried out under atmosphericconditions, the cap would need to be vented.

However once the centrifuge rotor accelerates, the angle of theswing-out buckets progressively changes the orientation of the cups 34a,34b and of the components 10a and 10b to the position shown in FIG. 5b.Once the centrifuge reaches a threshold speed when a centrifugal forcetypically of 100g is generated, the resin will flow through the openings36 into the components as shown in FIG. 5c. Once all the resin hasflowed into the components as shown in FIG. 5d, the speed of the rotoris gradually reduced to zero and the angle of the swing-out buckets isprogressively reduced in a controlled fashion to the rest position shownin FIG. 5e, to ensure that the resin does not run out of the componentas the centrifuge slows down.

It is an important feature of the invention that the quantity of resincharged into each cup 34 is that quantity required to precisely fill thecomponent 10 to a desired level. If the metering of resin into the cupis carried out at atmospheric pressure, then considerable meteringaccuracy is possible. Alternatively however the exact quantity of resincan be introduced by dimensioning each cup 34 so that it will holdexactly the correct amount of resin, and then using a doctor blade 40 towipe across the top of each cup to produce a uniform level of filling,as shown in FIGS. 6a and 6b.

In another embodiment of apparatus in accordance with the invention, thecentrifuge has four relatively rotatable rings. A first ring 42 carriesthe components 10. A second ring 44 is divided to define chambers 46(FIG. 7a) of specified dimensions to accommodate a precise volume ofresin. The ring 44 can be moved relative to a third ring 48 between twopositions to either admit resin from a central reservoir 50 to thechambers 46, or to close the chambers so that no resin can be admitted.

The apparatus also includes a fourth ring 52 in which dispensingopenings 54 are formed opposite the location of components 10.

During the initial operation of the centrifuge, as shown in FIG. 7a, thering 44 is moved so that resin can flow into the chambers 46.

In a second stage, shown in FIG. 7b the ring 44 is moved so that thechambers 46 are closed and a defined volume of resin is trapped therein.

In a third stage the ring 44 is moved until the chambers 46 line up withthe dispensing openings 54, and in this position the centrifugal forcearising from centrifuge rotation results in the resin being forced outof the chambers into the components 10, in a manner similar to thatalready described.

The invention thus allows accurate and rapid filling of electroniccomponents with a settable resin. As already pointed out however theinvention is not limited to this application.

It is a particular advantage of the present invention that the cycletime for the filling of each component is significantly less than thatrequired by prior art methods.

I claim:
 1. A method of encapsulating components in a housing with anencapsulating liquid, the method comprising the steps ofa. locatingcomponents in respective housings, each housing having walls defining acavity with an open top, b. providing a separate dispensing vessel witha dispensing opening for each housing, c. placing into each dispensingvessel the quantity of liquid required to encapsulate the components inthe associated housing, d. placing the housings and the dispensingvessels into a centrifuge so that the opening of each housing, when thecentrifuge is operating, lies radially outside of a dispensing openingof the dispensing vessel, with the housing walls surrounding each cavityopening being out of contact with the dispensing vessel, e. evacuatingthe centrifuge so that air is evacuated from the cavity before theliquid is introduced, f. operating the centrifuge so that all the liquidflows out of the dispensing opening, into the cavity and around thecomponent through centrifugal force.
 2. A method as claimed in claim 1,wherein the liquid is degassed before being placed in the dispensingvessel.
 3. A method as claimed in claim 1, wherein the liquid is anencapsulating resin.
 4. A method as claimed in claim 2, wherein theliquid is an encapsulating resin.
 5. A method as claimed in claim 1,wherein the step of placing into each dispensing vessel the quantity ofliquid, required to encapsulate the components in the associated housingtakes place under atmospheric conditions and the centrifuging operationtakes place under vacuum conditions.
 6. A method as claimed in claim 2,wherein the step of placing into each dispensing vessel the quantity ofliquid required to encapsulate the components in the associated housingtakes place under atmospheric conditions and the centrifuging operationtakes place under vacuum conditions.
 7. A method as claimed in claim 3,wherein the step of placing into each dispensing vessel the quantity ofliquid required to encapsulate the components in the associated housingtakes place under atmospheric conditions and the centrifuging operationtakes place under vacuum conditions.
 8. A method as claimed in claim 4,wherein the step of placing into each dispensing vessel the quantity ofliquid required to encapsulate the components in the associated housingtakes place under atmospheric conditions and the centrifuging operationtakes place under vacuum conditions.
 9. Apparatus for encapsulatingcomponents in an encapsulating liquid, the apparatus comprising aplurality of dispensing vessels each having a dispensing opening, thedimensions of the dispensing openings being such that encapsulatingliquid will not flow therethrough under normal gravitational conditionsbut will flow therethrough when centrifugal force is applied, a vacuumcentrifuge, means for supporting a plurality of open-topped housingsdefining cavities within which the components will be encapsulated, andmeans for locating the dispensing vessels in the vacuum centrifuge sothat when the open-topped housings are in position and the centrifuge isoperating the cavities in the housings are evacuated and the open topsof the housings are located radially outward of, and out of contactwith, the dispensing openings of the dispensing vessels such that whenthe liquid is drawn out of the dispensing openings on application ofcentrifugal force through centrifuge operation, the liquid will flowinto the cavities and around the components in the cavities toencapsulate the components.
 10. Apparatus as claimed in claim 9, whereinthe dispensing vessel comprises a cup with an open top and a bore in itsbase to form the dispensing opening.
 11. Apparatus as claimed in claim9, wherein the dispensing vessels and the supporting means for theopen-topped housings are mounted together in swing-out buckets so thatthe open tops of the dispensing vessels are uppermost until thecentrifuge starts to operate.
 12. Apparatus as claimed in claim 10,wherein the dispensing vessels and the supporting means for theopen-topped housings are mounted together in swing-out buckets so thatthe open tops of the dispensing vessels are uppermost until thecentrifuge starts to operate.
 13. Encapsulating apparatus forencapsulating components located in housings with a viscous liquidwherein the housings each have an open face, the components are locatedin the housings but do not occupy all the internal space in the housing,and the unoccupied internal space is to be filled with the viscousliquid to encapsulate the component, the apparatus comprising:a vacuumcentrifuge; housing locating means for locating the housings in thecentrifuge; a plurality of dispensing vessels with the number ofdispensing vessels being the same as the number of housings, thedispensing vessels each having an internal volume equal to the volume ofthe unoccupied internal space to be filled in the correspondinghousings; each dispensing vessel having a filling opening and adispensing opening; providing a cage for supporting the dispensingvessels, and the housing locating means in the centrifuge so that: whenthe centrifuge is not operating, the dispensing vessels are positionedwith their filling openings uppermost and when the centrifuge isoperating, the dispensing vessels are positioned with their dispensingopenings facing radially outward and the housing locating means arepositioned so that the housings held by the locating means are locatedaligned with, but out of contact with the dispensing openings of thecorresponding dispensing vessels.
 14. Apparatus as claimed in claim 13,wherein the dispensing vessel comprises a cup with an open top and abore in its base to form the dispensing opening.
 15. Apparatus asclaimed in claim 13, wherein the dispensing vessels and the supportingmeans for the open-topped housings are mounted together in swing-outbuckets so that the open tops of the dispensing vessels are uppermostuntil the centrifuge starts to operate.
 16. A method of encapsulatingcomponents located in housings with a viscous liquid wherein thehousings each have an open face, each component is located in a housingbut does not occupy all the internal space in the housing, and theunoccupied internal space is to be filled with the viscous liquid toencapsulate the component, the method comprising the steps of:placingeach component in a housing; locating the housings in a vacuumcentrifuge; filling with viscous liquid a dispensing vessel which has afilling opening, and a dispensing opening, and the volume of liquidfilled into each vessel being substantially the same as the volume ofthe unoccupied internal space to be filled in the housing; locating thedispensing vessels in the vacuum centrifuge with each dispensing vesselcorresponding with a single housing and being spaced from thecorresponding housing; evacuating the centrifuge; operating thecentrifuge so that liquid is drawn out of the dispensing vessel bycentrifugal force to enter the housing; continuing centrifuge operatinguntil all the liquid has been drawn from each dispensing vessel and hasentered the corresponding housing; and stopping the centrifuge andremoving the housings.
 17. A method as claimed in claim 16, wherein theliquid is degassed before being placed in the dispensing vessel.
 18. Amethod as claimed in claim 16, wherein the liquid is an encapsulatingresin.
 19. A method as claimed in claim 16, wherein the step of placinginto each dispensing vessel the quantity of liquid required toencapsulate the components in the associated housing takes place underatmospheric conditions and the centrifuging operation takes place undervacuum conditions.